Bacillus anthracis is a Gram-positive, aerobic, spore forming bacterium that is responsible for the deadly disease anthrax. There are three recognized routes of anthrax infection including cutaneous (through skin), gastrointestinal, and pulmonary (via inhalation) infection. Of the three ways to contract the disease, inhalation is the avenue that most frequently leads to the death of the patient.
Anthrax secretes a deadly three-component exotoxin which is comprised of three proteins, lethal factor (LF), edema factor (EF), and protective antigen (PA). The anthrax toxin is a bipartite toxin that contains A and B moieties, similar to that of diphtheria toxin and many clostridial toxins. The LF and EF proteins function as enzymatic A moieties of the toxin, while the PA protein functions as the B, or binding, moiety.
During the process of intoxication, PA binds to its cell surface receptor, (e.g., anthrax receptor (ATR) and/or capillary morphogenesis gene 2 (CMG2)) and is cleaved at the sequence RKKR (residues 193-196 of SEQ ID NO:2) by cell surface proteases such as furin. This cleavage releases a 20 kilodalton fragment of the PA protein, leaving a 63 kilodalton fragment of the PA protein bound to the cell surface (PA63). Some cleavage to the PA63 form may be mediated by serum proteases and occur prior to PA, in this case PA63, binding to the cell surface. Release of the 20 kilodalton PA fragment enables the PA63 fragment to multimerize into a heptameric ring structure and exposes a site on PA63 to which LF and EF bind with high affinity. The complex is then internalized by receptor-mediated endocytosis. Acidification of the vesicle causes conformational changes in the pA63 heptamer that result in transportation of LF and EF toxins across the endosomal membrane, after which they are released into the cytosol where they exert their cytotoxic effects. The edema factor (EF) component of edema toxin (EF+PA) is a calmodulin dependent adenylate cyclase whose action upsets cellular water homeostasis mechanisms, thereby resulting in swelling of infected tissues. The lethal factor (LF) moiety of lethal toxin (LF+PA) is a zinc metalloproteinase that inactivates mitogen activated protein kinase kinase in vitro. Lethal factor induces a hyperinflammatory condition in macrophages resulting in the production of proinflammatory cytokines including TNF-alpha and interleukin-1beta, which are responsible for shock and death of anthrax patients. For more detailed reviews of Bacillus Anthracis infection and anthrax toxin please see, e.g., Critical Reviews in Microbiology (2001) 27:167-200, Medical Progress (1999) 341:815-826, and Microbes and Infection (1999) 2:131-139, each of which are hereby incorporated by reference in their entireties.
There is a clear need, therefore, for identification and characterization of compositions, such as antibodies, that influence the biological activity of anthrax toxins.